In mobile communications, ARQ (Automatic Repeat Request) is applied to downlink data from a radio communication base station apparatus (hereinafter abbreviated to “base station”) to a radio communication mobile station apparatus (hereinafter abbreviated to “mobile station”). That is to say, a mobile station feeds back a response signal representing error detection results of downlink data, to the base station. A mobile station performs a CRC (Cyclic Redundancy Check) on downlink data, and, if CRC=OK is found (no error), feed back an ACK (ACKnowledgement), and, if CRC=NG is found (error present), feed back a NACK (Negative ACKnowledgement), as a response signal to the base station. This response signal is transmitted to the base station using an uplink control channel such as a PUCCH (Physical Uplink Control Channel), for example.
Furthermore, a base station transmits control information for reporting a downlink data resource allocation result to a mobile station. This control information is transmitted to a mobile station using a downlink control channel such as an L1/L2CCH (L1/L2 Control Channel), for example. Each L1/L2CCH occupies one or a plurality of CCEs (Control Channel Elements). When one L1/L2CCH occupies a plurality of CCEs, one L1/L2CCH occupies a consecutive plurality of CCEs. The base station allocates an L1/L2CCH from among a plurality of L1/L2CCHs for each mobile station in accordance with the number of CCEs necessary for carrying control information, and transmits control information mapped on a physical resource corresponding to a CCE occupied by each L1/L2CCH.
In order to use downlink communication resources efficiently, mutually mapping between CCE's and PUCCH's has been investigated. Each mobile station can determine a PUCCH to be used for transmission of a response signal from that mobile station from a CCE corresponding to a physical resource to which control information for that mobile station is mapped in accordance with this mapping.
Also, investigation has been carried out into code-multiplexing a plurality of response signals from a plurality of mobile stations by means of spreading using a ZC (Zadoff-Chu) sequence and Walsh sequence, as shown in FIG. 1 (see Non-Patent Document 1). In FIG. 1, (W0, W1, W2, W3) represents a Walsh sequence with a sequence length of four. As shown in FIG. 1, in a mobile station, first, a response signal of ACK or NACK is subject to first spreading to one symbol by a ZC sequence (with a sequence length of twelve) in the frequency domain. Then a response signal subjected to first spreading is subject to an IFFT (Inverse Fast Fourier Transform) in association with W0 to W3. A response signal that has been spread in the frequency domain by a ZC sequence with a sequence length of twelve is transformed to a time-domain ZC sequence with a sequence length of twelve by this IFFT. Then this signal subjected to the IFFT is subject to second spreading using a Walsh sequence (with a sequence length of four). That is to say, one response signal is arranged in four symbols S0 through S3. Response signal spreading is also performed in a similar way in other mobile stations using a ZC sequence and Walsh sequence. However, different mobile stations use ZC sequences with mutually different Cyclic Shift values in the time domain, or mutually different Walsh sequences. Here, since the time-domain sequence length of a ZC sequence is twelve, it is possible to use twelve ZC sequences with cyclic shift values of 0 through 11 generated from the same ZC sequence. Also, since the sequence length of a Walsh sequence is four, four mutually different Walsh sequences can be used. Therefore, in an ideal communication environment, response signals from a maximum of forty eight (12×4) mobile stations can be code-multiplexed.
Here, cross-correlation between ZC sequences with mutually different cyclic shift values generated from the same ZC sequence is 0. Therefore, in an ideal communication environment, as shown in FIG. 2, a plurality of code-multiplexed response signals spread by ZC sequences with mutually different cyclic shift values (cyclic shift values of 0 through 11) can be separated without inter-code interference in the time domain by correlation processing in the base station.
In the case of the 3GPP LTE (3rd Generation Partnership Project Long Term Evolution) PUCCH, a CQI (Channel Quality Indicator) signal is code-multiplexed as well as the above-described ACK/NACK signals. While an ACK/NACK signal is 1-symbol information, as shown in FIG. 1, a CQI signal is 5-symbol information. As shown in FIG. 3, a mobile station spreads a CQI signal by a ZC sequence with a sequence length of twelve and cyclic shift value P, and transmits the spread CQI signal after performing IFFT processing. Since a Walsh sequence is not applied to a CQI signal, a Walsh sequence cannot be used in the base station for separation of an ACK/NACK signal and CQI signal. Thus, by performing despreading by a ZC sequence of an ACK/NACK signal and CQI signal spread by ZC sequences corresponding to different cyclic shifts, a base station can separate the ACK/NACK signal and CQI signal with almost no inter-code interference.
However, due to an influence of transmission timing difference in mobile station, multipath delayed waves, frequency offset, and so forth, a plurality of ACK/NACK signals and CQI signals from a plurality of mobile stations do not necessarily reach a base station at the same time. To take the case of an ACK/NACK signal as an example, as shown in FIG. 4, if the transmission timing of an ACK/NACK signal spread by a ZC sequence with a cyclic shift value of 0 is delayed from the correct transmission timing, the correlation peak of the ZC sequence with a cyclic shift value of 0 appears in the detection window of a ZC sequence with a cyclic shift value of 1. Also, as shown in FIG. 5, if there is a delayed wave in an ACK/NACK spread by a ZC sequence with a cyclic shift value of 0, interference leakage due to that delayed wave appears in the detection window of a ZC sequence with a cyclic shift value of 1. That is to say, in these cases, a ZC sequence with a cyclic shift value of 1 receives interference from a ZC sequence with a cyclic shift value of 0. Therefore, in these cases, separability of an ACK/NACK signal spread by a ZC sequence with a cyclic shift value of 0 and an ACK/NACK signal spread by a ZC sequence with a cyclic shift value of 1 degrades. That is to say, if ZC sequences with mutually adjacent cyclic shift values are used, there is a possibility of ACK/NACK signal separability degrading.
Thus, heretofore, when performing code multiplexing of a plurality of response signals by ZC sequence spreading, a cyclic shift value difference (cyclic shift interval) has been provided between ZC sequences that is sufficient to prevent the occurrence of inter-code interference between ZC sequences. For example, the cyclic shift value difference between ZC sequences is made 2, and of twelve ZC sequences with cyclic shift values of 0 through 11, only the six ZC sequences corresponding to cyclic shift values 0, 2, 4, 6, 8, and 10 are used for first spreading of a response signal. Therefore, when using a Walsh sequence with a sequence length of four for second spreading of a response signal, response signals from a maximum of twenty four (6×4) mobile stations can be code-multiplexed.
In Non-Patent Document 2, an example is disclosed in which, on a response signal from a mobile station, first spreading is performed using six ZC sequences with cyclic shift values 0, 2, 4, 6, 8, and 10, and second spreading is performed using Walsh sequences with sequence length of four. FIG. 6 is a drawing showing, by a mesh structure, an arrangement of CCEs that can be allocated to mobile stations for ACK/NACK signal transmission use (hereinafter abbreviated to “ACK/NACK use”). Here, it is assumed that a CCE number and a PUCCH number defined by a ZC sequence cyclic shift value and Walsh sequence number are mapped on a one-to-one basis. That is to say, it is assumed that CCE #1 and PUCCH #1, CCE #2 and PUCCH #2, CCE #3 and PUCCH #3, and so on, are mutually mapped (the same applying subsequently). In FIG. 6, the horizontal axis indicates a ZC sequence cyclic shift value, and the vertical axis indicates a Walsh sequence number. Since inter-code interference is extremely unlikely to occur between Walsh sequences #0 and #2, as shown in FIG. 6 ZC sequences with the same cyclic shift values are used for CCEs subjected to second spreading by Walsh sequence #0 and CCEs subjected to second spreading by Walsh sequence #2.    Non-patent Document 1: Multiplexing capability of CQIs and ACK/NACKs from different UEs (ftp://ftp.3gpp.org/TSG_RAN/WG1_RL1/TSGR1—49/Docs/R1-072315.zip)    Non-patent Document 2: Signaling of Implicit ACK/NACK resources (ftp://ftp.3gpp.org/TSG_RAN/WG1_RL1/TSGR1—49/Docs/R1-073006.zip)